Communication systems currently put into practice gives high importance to a technique of compensating for data errors occurring on a transmission path between a transmitter and a receiver, which technique is exemplified by Automatic Repeat request (ARQ).
In the scheme of ARQ, a receiver that receives data (e.g. packets) detects the presence or the absence of an error in the data received from a transmitter through the use of an error detection code, such as a CRC (Cyclic Redundancy Check) code, attached to the received data.
As a result of the detection, if no error is detected in the received data, the receiver returns an ACK (ACKnowledgement) signal to the transmitter to thereby request the transmitter to transmit next new data to the receiver.
On the other hand, if an error is detected in the received data, the receiver returns a NACK (Negative ACKnowledgement) signal to the transmitter to thereby request the transmitter to retransmit retransmission data to the receiver.
This reduces the influence caused by error in data on the transmission path, realizing highly reliable data transmission.
However, in some cases of the ARQ scheme, data retransmission is repeated. One of the solutions to limit the number of times of retransmitting data is HARQ (Hybrid ARQ) scheme.
As illustrated in FIG. 1, in the HARQ scheme, the transmitter (e.g., abase station) first transmits new data (e.g., data destined for a terminal) to the receiver (e.g., a terminal) (step S100). The receiver receives the data (step S101) and detects the presence or the absence of an error in the received data (step S102).
As a result of the error detection, an error is detected in the received data, the receiver transmits a NACK signal to the transmitter (step S104) and retains the received data (step S103).
Next, upon receipt of the NACK signal from the receiver, the transmitter transmits retransmission data corresponding to (e.g., the same as) data transmitted for the last time to the receiver (step S105).
Upon receipt of the retransmission data from the transmitter (step S106), the receiver synthesizes the previously-received data stored in the receiver and the retransmitted data and decodes the synthesized data (step S107). Examples of synthesis in the HARQ technique are Chase Combining, and IR (Incremental Redundancy) Combining.
Then, the receiver detects the presence or the absence of an error in the decoded data (step S108) and transmits an ACK or NACK signal to the transmitter (step S109) in accordance with the result of the error detection.
As the above, in the HARQ scheme, the receiver synthesizes (retransmission synthesis) data previously received and retransmission data and decodes the synthesized data, so that the SIR (Signal-to-Interference Ratio) of the data can be improved, realizing good reception characteristics.
Consequently, HARQ can limit the number of times of retransmitting data.
In one of known techniques, a transmitter forcibly retransmits the same packet to a receiver before the RTT (Round Trip Time) after transmission of the same packet expires and concurrently irrespective of the response of the reception of the packet from the receiver (Patent Document 1).    [Patent Document 1] WO 2007/007383
In a communication system adopting the ARQ or HARQ scheme, when the transmitter transmits retransmission data to the receiver, the transmitter ensures communication resource (e.g. time, frequency, code, and hardware such as antenna) for the data retransmission.
Ensuring of communication resource for retransmission data may cause a lack of communication resource to transmit data (e.g., new data destined for a different terminal from one requesting the retransmission data) except for such retransmission data.
Consequently, there is a possibility of lowering efficiency in using communication resource.